1. Field of Invention
The present invention relates to a planar antenna structure, and more particularly to a planar E-inverted antenna structure.
2. Related Art
An antenna structure is usually used to transmit and receive electromagnetic waves. To this purpose, specific designs are usually required to be able to irradiate signals in electromagnetic waveforms of radio frequency from the transmitter through the air medium, and/or to intercept electromagnetic waves carried by air media and convert it to meaningful radio-frequency signals. Antenna design is therefore an important in the actual performance of wireless equipment.
Presently, planar F-inverted antennae are known in the art, called like this because of its F-inverted shape. The operating length of the planar F-inverted antenna is only about λ/4, while that of the grounding area is λ/2. On the other hand, because the planar F-inverted antenna only uses a metallic conductor in association with adequate feeding and antenna short-circuits to the grounding area, its manufacturing cost is relatively low, and it can be soldered to the printed circuit board.
FIG. 1 illustrates a planar F-inverted antenna known in the art. The known antenna includes an open-circuit area 11, a short-circuit end 12, and a grounding area 13. A signal line 14 connects the open-circuit area 11 to the grounding area 13. A radio-frequency signal travels via the signal line 14 to the open-circuit area 11, or from the open-circuit area 11 to the signal line 14. With the short-circuit end 12 connected to the grounding area 13, signals therefore travel from the open-circuit area 11 via the short-circuit end 12 to the grounding area 13.
The metallic conductor of the planar F-inverted antenna of the prior art can be either in a linear shape or flat shape. In FIG. 1, the metallic conductor is in a flat plate shape, and thereby can be soldered on the printed circuit board as a surface-mount device, being thereby used as a concealed type antenna.
To support the metallic conductor, e.g. the open-circuit area 11 in a manner not to short-circuit the grounding area 13, an insulating material 15 such as sponge material is usually inserted between the open-circuit area 11 and the grounding area 13 to support the open-circuit area.
Some problems may occur when a sponge material is used to support the open-circuit area 11 in the planar F-inverted antenna. The sponge or insulating material can absorb electromagnetic waves so that signal loss occurs at the transmission and reception. The antenna gain therefore is decreased. If sponge materials are used as support, additional and specific processing steps are needed, which increases manufacturing costs. As shown in FIG. 4, when the antenna is placed in a horizontal position, the radiation pattern further exhibits a central recessed area, and the radiation pattern therefore is not satisfactory. In addition, if the antenna has a baseband frequency f, its next harmonic is 3f, which is not suitable for double-baseband use, such as 900 MHz and 1800 MHz.